Ok, i was just having the same problem. Since you replaced the thermostat, did you "burp" the system? There is a bolt on the top-front of the engine (youll know its the vent, it says it right on there), with the engine running, open it very slowly until a little coolant comes out, then close it. ALso, what temp thermostat did you put in? I know I replaced a 195 with a 192, the 195 was old and it let water creep by. Make sure you bought a good one, mine costed $12. I also replaced the Coolant Temperature Sensor (NOT sender). After a few times of running the car from cold to hot, I get nice warm air. I think the main problem though is you have air in the system, get that out, and youll be fine.